1. Field of the Invention
This invention relates to a method for producing a thin-film photovoltaic cell having a cell level integrated bypass diode and to the photovoltaic cell produced thereby; and, in particular, to a method which employs one or more chemical etchant(s) to separate a portion of a photovoltaic cell and, by appropriate electrical interconnection, to utilize the separated portion to define a bypass diode for a cell.
2. Description of the Art
A photovoltaic cell converts solar radiant energy (sunlight) incident thereon into electrical energy as the result of the photovoltaic effect. Of particular recent interest is the large-scale and cost-effective conversion of solar radiation into electricity using arrays of photovoltaic cells assembled into photovoltaic panels.
A typical first generation photovoltaic panel involves first producing a large number of photovoltaic cells from thinly sliced substrates of single or polycrystalline silicon. Cells approximately fifteen centimeters square (15 cm×15 cm) are cut from silicon wafers a few hundred microns in thickness and are connected electrically in series to form strings. Multiple strings are further connected in series or in parallel, arranged on a supporting glass pane, and encapsulated with polymeric resin and film to form a photovoltaic module. The module is usually provided with a frame to form the photovoltaic panel.
A second generation photovoltaic panel involves utilizing a thin-film of semiconductor material, for example hydrogenated amorphous silicon (a-Si:H), as the active photovoltaic material. The a-Si:H film is deposited on a supporting superstrate in a glow discharge of silane gas. Resulting thin-film silicon devices exhibit solar conversion efficiencies in excess of ten percent (10%). A unit cell and the tandem cell or multi-junction amorphous silicon solar cell can only provide a small output voltage of up to a few volts (a single junction ˜0.9 V; tandem junction ˜1.6V; multi-junction, depending upon the number of junctions, more than 2 V). So, a number of solar cells are typically electrically interconnected in series to produce working voltages.
When all cells in an array are illuminated, each cell will be forward biased. However, if one or more of the cells is partially shaded or shadowed (i.e., not illuminated), such as by falling leaves, snow, or if there are physical differences in the cells such as caused by cell breakage, this mismatch in the properties of interconnected cells can create operating problems for series connected solar cells. This mismatch of different cell in output can dramatically decrease the output current of the entire module, and in some cases the mismatched unit cell merely functions as a load to cause heat generation or reverse bias. The excess heat or the strong reverse bias voltage may permanently damage the unit cell or possibly melt the encapsulate material.
To guard against such damage it is known to provide a protective bypass diode. One bypass diode may be connected across several cells, or, for enhanced reliability, each cell may have its own bypass diode connected in parallel and in an opposite direction thereto, thereby reducing the influence of such a mismatch. If the cells are working normally with fully illumination and producing energy, the bypass diodes are reverse biased and the current flow is through the cells. However, if any mismatch happens, the current flow through the cell becomes limited and reverse biased, the parallel-connected bypass diode becomes forward biased, and current flow is conducted through the bypass diode, thereby protecting the affected cell.
U.S. Pat. Pub. No. 2002/0164,834 (Boutros et al.) discloses a method for making a solar cell with an integrated bypass diode. The method comprises multiple steps of depositing layers with opposite type and different level of dopants on one surface layer of the solar cell to form a bypass diode.
U.S. Pat. No. 6,784,358 (Kukulka) shows a solar cell structure with a discrete amorphous silicon bypass diode. A discrete amorphous silicon bypass diode is supported on either the first or second metallization layer of the cell.
The above mentioned configurations, however, require additional semiconductor steps to incorporate the diode into the substrate. The approaches are complex and cause assembly difficulties, for example in the case of series connections, it is very complicated. Manufacturing cost increase commensurately.
Accordingly, in view of the foregoing, it is believed advantageous to provide an efficient method for production of photovoltaic panels with cell-level integrated bypass diodes with reduced costs.